1. Field of the Invention
This invention is related generally to an instrument for measuring parameters of an air flow, and particularly to the sensing of static pressure in atmospheric flows while compensating for wide variations in the angle of attack of the fluid flow relative to the sensing device.
2. Description of the Prior Art
Quantitative measurements of fluid flow are accomplished by a wide variety of instruments and techniques. The degree of accuracy required for a certain measurement depends on the application of the resulting information. While in some cases extreme accuracy is needed, only a rough estimate is required in other cases. In most industrial applications, even a slight error in measurements taken on large quantities of fluid can be very important.
Flow measurements frequently relate to the measurements of static or ambient pressure. Static pressure may be defined as pressure exerted by a fluid normal to stream lines of a flow of the fluid. This pressure will be constant across the flow in those situations where the formation streamlines are parallel to one another. If, for example, the streamlines are not parallel, but are curved, the static pressure will vary across the flow. A common technique for measuring the ambient or static pressure in a fluid stream relative to some reference pressure is by means of a pitot-static tube or probe. An aperture, generally referred to as a static pressure tap or a piezometer opening, is provided in the wall of the pitot-static tube. The pressure sensed by the aperture can be measured by any suitable device such as a manometer or a conventional pressure gauge.
The pitot-static tube measures the local and static pressure in a fluid flow by application of the principle of conservation of mechanical energy. This principle is applied by converting the velocity of the fluid flow into a pressure head. That is, if the fluid changes its velocity from V.sub.s1 to V.sub.s2 by application of a suitable force to the fluid flow, the pressure of the fluid will change from p.sub.s1 to p.sub.s2 in accordance with relationships known to those in the art. These relationships may be expressed by the equation: EQU V.sub.s2.sup.2 - V.sub.s1.sup.2 = (2g/d) .times. (p.sub.s1 - p.sub.s2),
where g is the acceleration due to gravity, and d is the density of the fluid.
If the pitot-static probe is not properly aligned with the flow, or if the flow is highly turbulent, the indicated static pressure tends to be lower than the true static pressure. Measuring barometric pressure under windy conditions, for example, necessitates mounting the pitot-static probe on a wind vane or similar device, to continuously align the probe with the oncoming wind. Further, problems often arise with the pressure ports becoming clogged with rain, snow, and the like.
U.S. Pat. No. 3,673,866, issued July 4, 1972 to Alperovich et al., discloses a pitot-static probe having an adjustable collar on the probe. Mounting the collar in front of the static pressure openings compensates for measuring errors where the static pressure is greater than the free-stream static pressure, while arranging the collar behind the openings compensates for errors encountered when the static pressure is lower than the free-stream static pressure. The static pressure measuring openings are exposed to rain, snow and the like, however, and may become clogged and variations in the angle of attack may cause errors which will not be compensated for the collar.
U.S. Pat. No. 3,443,431, issued May 13, 1969 to F. W. Hagen, discloses a static pressure sensing device intended to compensate for wide variations in the angle of attack between the device and associated flow stream. The measuring device has static pressure measuring openings along axes which are inclined forwardly from a position normal to the surface of the probe, rather than normal to the probe as is conventional. The openings are still exposed to the elements and are subject to clogging, and the probe would have to be mounted on a wind vane if it was to be used to measure barometric pressure under windy conditions.